College of Agricultural, Consumer and Environmental Sciences New Mexico State University

What's in this Guide

All soils should be tested for available plant nutrients before adding
fertilizer. Be sure to follow NMSU Extension Guide A-109,
Test Determine Soil Needs, when sampling to assure an accurate representation of your
soil conditions. If fertilizer is required, then follow this guide to determine
what might be best for your conditions (table 1) .

A fertilizer is any organic or inorganic material (or combination)
that is added to the soil to supply sufficient amounts of one or more elements
essential to the growth of plants. A profitable harvest will depend upon
choosing how, when, where, and what kind of fertilizer to apply. Recent
scrutiny over the effects of synthetic fertilizers on the environment,
poor fertilizer efficiency, high fertilizer prices, and low crop prices
have prompted the need for a rational approach to choosing fertilizers
in New Mexico. Additionally, a rational approach to fertilization includes
knowing plant needs, type of tillage, availability of equipment, short-
and long-term effects on the soil, and labor.

Plants require a minimum of 16 nutrients. Plants use nitrogen
(N), phosphorus (P), and potassium (K) in significant amounts, so they
must be replaced periodically to sustain productivity. Synthetic fertilizers
have been developed to supply the three most-used nutrients. Additionally,
several fertilizers have been developed to supply the other 13 nutrients
when needed. All materials sold as fertilizer in New Mexico must specify
the grade, or guaranteed analysis, of the material. Grades are stated
in terms of the minimum percentage of nitrogen (N), available phosphorus
as phosphoric acid (P2O5), and water-soluble potash
(K2O), in that order (table 1).Thus, a
bag of fertilizer that has the numbers 10-10-10 contains 10% nitrogen,
10% phosphorus as P2O5, and 10% potassium as K2O.
In other words, a 50-pound bag of fertilizer with the numbers 10-10-10
contains 5 pounds of N, 5 pounds of P2O5, and 5 pounds
of K2O.

Fertilizer Types

Current information on synthetic fertilizers is presented in table 1. A
complete fertilizer is a chemical compound (or blend) containing
significant quantities of the three primary fertilizer nutrients (N, P, and K).

Bulk-blended fertilizer is a physical mixture of dry, granular
fertilizer­ materials to produce specific fertilizer grades. Individual
granules in the bulk-blended fertilizer do not have the same ratio and
content of plant food as does the mixture as a whole.

Mixed fertilizer is composed of two or more fertilizer
materials blended or granulated together into individual mixes. The term
includes powders, granules, flakes, liquids, suspensions, and slurry mixtures.
Specific blends should be based on soil test results and crop needs
Test Determine Soil Needs).

Forms

In addition to the different types of synthetic fertilizers, there are
several physical forms by which they are marketed. The physical forms include
gas, dry solid, liquid, slurry, and suspension.

Gas. Anhydrous ammonia (NH3) is the only
gas fertilizer marketed in the United States, however, it is stored and
distributed as a compressed liquid. Liquid ammonia expands instantly to
a gas when it moves out of the application knives. Ammonia gas reacts with
water to form the ammonium ion (NH4+) in the soil
environment. Anhydrous ammonia is most popular in eastern New Mexico.

Dry solids. Dry solids are best used as a broadcast
fertilizer prior to planting. Three different kinds of dry solid fertilizers
are available in New Mexico: completely water soluble, dry mixes, and dry blends.

Completely water-soluble dry pellets are fertilizer nutrients that have
been chemically combined with water to form a solution and then evaporated,
leaving a uniform, dry solid.

Dry mix (bulk mix) are mixtures of pellets in which each pellet has only
one nutrient.

Dry blends are mixtures of pellets that have more than one nutrient associated
with them.

Liquid. A liquid fertilizer is a fluid in which the plant
nutrients are in true solution (wholly dissolved). Other liquid fertilizers
that can be handled as liquids include suspensions and slurries. Because
liquids can be applied through a planter and combined with herbicides,
they have become quite popular. Liquids are best suited to applications
where the material can be injected through knives attached to tillage implements
or injected into irrigation water (see California Fertilizer Association's
Western Fertilizer Handbook).

Slurries are combinations of liquid and dry forms. The solid particles
are prevented from settling by constant stirring within the tank.

Suspensions are fluid fertilizers containing dissolved and undissolved
plant nutrients. The undissolved plant nutrients are kept in suspension
with a suspending agent. The suspension must be flowable enough to be mixed,
pumped, agitated, and applied to the soil in a homogeneous mixture. Agitation
is not always required.

Incorporating Fertilizers

Most fertilizers should be incorporated either with tillage or irrigation
water. Incorporation reduces nitrogen losses due to volatilization (loss
as a gas) and places the nutrients in the crop root zone.

Conventional tillage systems that mix the top of the soil profile
will incorporate surface-applied fertilizer. No-till and minimum-till operations
will not incorporate fertilizer into the crop root zone. Tillage systems
affect fertilizer management decisions because some nutrients (such as
phosphorus and potassium) do not readily move through the soil to the crop
root zone where the nutrient can be taken up by the plant.

Nitrogen management can also be affected by the tillage system.
For example, use of anhydrous ammonia requires that the slit left by the
injection knife be completely closed to prevent loss of gaseous nitrogen.
Reduced-till and no-till systems also require that the slit left by an
ammonia application be completely closed. Also, urea-containing fertilizers
should be incorporated to reduce loss of gaseous nitrogen.

Drip irrigation systems are also ideal systems for placing fertilizer
close to the root system. Depending on the depth of the drip tape, fertilizer
may also be broadcast and incorporated.

Application Methods

The best method of fertilizer application depends on the crop, available
equipment, fertilizer-pesticide combination, labor, irrigation and tillage
practices, and type of fertilizer. A major goal of many producers is to
improve fertilizer efficiency (that is, greater crop yield per unit of
fertilizer applied). The choice of method, however, also must meet future
agronomic and environmental requirements.

Broadcasting is the surface application of fertilizer
to the soil either before or after the crop has been established. The broadcast
application may or may not be incorporated.

Deep placement is also referred to as dual placement,
knife injection, and root-zone banding. Generally, with this method a mixture
of nitrogen and phosphorous (sometimes N-P-K) is injected into the soil
at a depth of 4-8 inches. The knife spacing is usually 15-30 inches (spacing
depends on row spacing).

Dribble is the surface application of fertilizer
to the soil, but differs from broadcast because it is applied in bands
(for example, 30-inch spacings). Often this method is used when the producer
is using a urea-ammonium nitrate solution fertilizer.

Foliar application is the placement of a dilute
fertilizer solution on the leaves of the crop. Foliar applications are
used when small amounts are needed and can be absorbed through the leaves.

Pop-up or starter refers to fertilizer
placed in small amounts in direct contact with the seed. Starter fertilizers
generally have a grade of 1-4-2 and are applied at 10-15 lb/acre.

Pre-plant fertilizers are applied during tillage
preparation and often are incorporated into the soil. Some herbicides can
be mixed with pre-plant fertilizers.

Post-emergent applications occur after the crop
has emerged from the soil. This is an efficient method for applying nitrogen
fertilizers because the nitrogen is applied just prior to the period of
greatest demand. Drip, center pivot, and linear move irrigation systems
may be the most efficient ways to apply post-emergent fertilizer.

Row refers to the placement of fertilizer in a concentrated
band below and to the side of the seed. In addition, this method includes
side-dressing, which is the application of fertilizer to the side of young
roots after crop emergence.

Strip placement generally refers to the application
of phosphorus and potassium fertilizers in narrow bands on the soil surface.
Often this method is used with mold board plowing practices in order to
distribute a band of fertilizer throughout the plow layer.

Variable rate (site-specific) applications refer
to the use of precision farming technology (using personal computers and
technologies such as geographic information software and global positioning
satellites) to alter the rate of fertilizer application as the equipment
travels across the field.

Changing Soil pH

It is occasionally necessary to increase soil acidity. Acidification properties
of fertilizers should be considered in fertilizer choice because of the
high pH conditions of New Mexico soils. The elements used to reduce soil
pH are elemental sulfur, sulfuric acid, aluminum sulfate, iron sulfate,
and ammonium polysulfide. Ammonium sulfate, ammonium phosphate, and similar
compounds have been considered effective in short-term decreases in pH,
but have no real long-term effects on pH. Short-term and localized reductions
in pH are more common for sodium-affected soils where sulfur replaces the
sodium (table 2).

Salt Index

Injury can result from synthetic nitrogen, phosphorous, and potassium fertilizers
applied at high rates, which increase the salt concentration in the soil
solution near the seed or the young root system. A salt index was developed
to compare the "saltiness" of the synthetic fertilizers. The index is the
ratio (5:100) of the decrease in osmotic potential of a solution containing
a fertilizer to that produced by the same weight of CaNO3. In
other words, a salt index (table 2) indicates the probability of fertilizer
burn from fertilizer application. Therefore, fertilizers with a low salt
index are generally considered to have a lower risk of injury when applied
properly. However, although higher grade fertilizers have a high salt index,
it takes less of a high grade fertilizer to supply the same amount of nutrient
as a low grade fertilizer. Therefore, on a per-nutrient basis, the salt
index is generally lower for high grade fertilizers (table
3).

Table 1. Types and characteristics of synthetic nitrogen fertilizers.

Fertilizer

Chemical formula

% N, P, K

Form

Application method

Advantages

Disadvantages

Ammonium nitrate

NH4NO3

34% N

Dry pellets

Broadcast or sidedressed.

NO3 is immediately available. Among dry fertilizers, second
only to urea in percentage N.

NO3 (1/2 of the N it contains) is leachable, and also subject
to denitrification in warm, wet soil. Hardens upon exposure to air.

Ammonium phosphate

NH4H2PO4

11% N
21% P

Dry granules

Broadcast or drilled.

Phosphorus is completely water soluble. Desirable N to P ratio for
row fertilizer, especially where K is not needed.

High residual acidity.

Ammonium phosphate and ammonium sulfate

NH4H2PO4(NH4)2SO4

16% N
9% P

Dry granules

Broadcast or drilled.

See ammonium phosphate and ammonium sulfate.

Ammonium polyphosphate

(NH4)(H3P2O7)

10% N
15% P

Liquid

Broadcast or drilled.

Completely water soluble source of P.

Cannot come in contact with seed.

Ammonium polyphosphate

(NH4)(H3P2O7)

15% N
27% P

Dry

Broadcast or drilled.

Completely water soluble source of P.

Cannot come in contact with seed.

Ammonium sulfate

(NH4)2SO4

21% N

Dry granules or pellets

Broadcast or sidedressed.

Acidifying effect desirable on alkaline soils.

Low analysis. Medium price. Highest acidifying effect of N sources.

Anhydrous ammonia

NH3

82% N

Compressed gas, 204 lb per square inch at 104¡F

Injecting 6" or deeper in moist but not saturated soil.

Low price. Non-leachable.

Requires high-pressure storage and applicators.

Aqua ammonia

NH4OH in water

21% N

Liquid

Inject at least 1 inch (2.5 cm) or deeper below soil surface.

Low price.

Must be covered to avoid ammonia escaping.

Calcium cyanamide

CaCN2

21% N

Black, dry pellets

Broadcast or sidedressed.

Can be used as an herbicide and as a defoliant (powdered form).

High price, intermediate reaction products are toxic.

Calcium nitrate

Ca(NO3)2

16% N

Dry granules

Broadcast or sidedressed.

Same as sodium nitrate.

Diammonium phosphate

(NH4)2HPO4

18 to 21% N
20 to 23% P

Dry granules

Broadcast or drilled.

Low price, completely water soluble source of P.

Cannot come in contact with seed.

Nitrogen solutions (Two are listed.)

Urea in ammonium nitrate solution

NH3 in ammonium nitrate solution

30% N

41% N

Liquid

Liquid

Sprayed or sidedressed.

Injected 1 to 2 deep preplant or sidedressed.

No pressure equipment needed.

Non-leachable until changed to NO3.

Some risk of loss of urea portion if not incorporated into the soil
by tillage or precipitation.
Slight vapor pressure of NH3. Must be covered 1 to
2 inches to prevent loss. Cannot come in contact with seed.

Potassium nitrate

KNO3

14% N
38% K

Dry granules

Broadcast or sidedressed.

Sodium nitrate

NaNO3

16% N

Dry granules

Broadcast or sidedressed.

Immediately available and non-acid forming.

Very high price, and sodium can raise pH.

Urea

CO(NH2)2

46% N

Dry pellets

Broadcast, sidedress, add to solutions, and liquid spray
on some crops.

High solubility. Non-leachable after it is converted to
NH4.

Leachable by rain shortly after application. Some risk of
loss of NH3 to atmosphere if not incorporated into the soil
by tillage or precipitation.

Ureaform

CO(NH2)2 combined with formaldehyde

38% N

Dry granules

Broadcast or sidedressed

Nitrogen is released over several weeks rather than all being available
immediately.

High price for field crops. Variable rate of N release. Depends somewhat
on moisture and temperature.

Glossary

analysis fertilizer - The percent composition of
a fertilizer as determined in a laboratory and expressed as total nitrogen
(N), available phosphoric acid (P2O5 ), and water-soluble
potash (K2O).

blended fertilizer - A mechanical mixture of different
fertilizer materials.

bulk-blended fertilizer - A physical mixture of
dry granular fertilizer materials to produce specific fertilizer ratios
and grades. Individual granules in the bulk-blended fertilizer do not have
the same ratio and content of plant food as does the mixture as a whole.

complete fertilizer - A chemical compound or a blend
of compounds containing significant quantities of nitrogen, phosphorous,
and potassium. It may contain other plant nutrients.

compound fertilizer
- A fertilizer formulated with two or more plant nutrients.

controlled-release fertilizer - Also called delayed
released, slow release, controlled availability, slow acting, and metered
release. A controlled dissolution of fertilizer at a lower rate than conventional
water-soluble fertilizers. Controlled-release properties may result from
coatings on water-soluble fertilizer or from low dissolution and/or mineralization
rates of fertilizer materials in soil.

fertilizer - Any organic or inorganic material of
natural or synthetic origin (other than liming material) that is added
to a soil to supply one or more elements essential to the growth of plants.

fertilizer grade - The guaranteed minimum analysis
in percent of the major plant nutrient elements contained in a fertilizer
material or in a mixed fertilizer. The analysis is usually designated as
N-P2O5 -K2O, but it may be N-P-K where
permitted or required as specified by state law. Grades must be expressed
in percent N-P-K for Soil Science Society of America publications (oxide
values may be included in parentheses).

fertilizer ratio- The relative proportions of primary
nutrients in a fertilizer grade divided by the highest common denominator
for that grade. For example, grades 10-6-4 and 20-12-8 have a ratio of
5-3-2.

fertilizer requirement - The quantity of certain
plant nutrients needed, in addition to the amount supplied by the soil,
to increase plant growth to a designated level.

fertilizer salt-index - The ratio of the decrease
in osmotic potential of a solution containing a fertilizer compound or
mixture to that produced by the same weight of NaNO3 X 100.

fluid fertilizer - Fertilizer wholly or partially
in solution that can be handled as a liquid, including clear liquids and
liquids containing solids in suspension.

foliar fertilization - Application of a dilute solution
of fertilizer nutrients to plant foliage; usually applied to supplement
nutrients absorbed by plant roots.

granular fertilizer - Fertilizer in the form of
particles sized between an upper and lower limit or between two screen
sizes, usually within the range of 1 to 4 mm and often more closely sized.
The desired size may be obtained by agglomerating smaller particles, crushing
and screening larger particles, controlling size in crystallization processes,
or prilling.

inorganic fertilizer - A fertilizer material in
which carbon is not an essential component of its basic chemical structure.
Urea is often considered an inorganic fertilizer because of its rapid hydrolysis
to form ammonium ions in soil.

mixed fertilizer - Two or more fertilizer materials
blended or granulated together into individual mixes. The term includes
dry mix powders as well as granulated, clear liquid, suspension, and slurry
mixtures.

organic fertilizer - A material containing carbon
and one or more plant nutrients in addition to hydrogen and/or oxygen.
Urea is often considered an inorganic fertilizer because of its rapid hydrolysis
to form ammonium ions in soil.

pop-up fertilizer - Fertilizer placed in small amounts
in direct contact with the seed.

side-dressed fertilizer - Application made to the
side of crop rows after plant emergence.

slow-release fertilizer - A fertilizer applied in
relatively small amounts with or near the seed; the nutrient is not immediately
available for the crops, but is released slowly over time.

soil fertility - The ability of a soil to supply
the nutrients essential to plant growth.

suspension fertilizer - A fluid fertilizer containing
dissolved and undissolved plant nutrients. The undissolved plant nutrients
are kept in suspension with a suspending agent, usually a swelling type
clay. The suspension must be flowable enough to be mixed, pumped, agitated,
and applied to the soil in a homogeneous mixture.

top-dressed fertilizer - The surface application
of fertilizer to a soil after the crop has been established.